This study will assess the utility of fluoxetine, a selective serotonin reuptake blocker, in both the acute and maintenance treatment of panic disorder and panic disorder with agoraphobia. Panic disorder is common, has an early onset, and may be chronic or episodic. Most patients develop secondary anticipatory anxiety. Many also develop significant agoraphobia and severely curtail their occupational and social functioning. Clinically, established treatments for panic disorder include tricyclic antidepressants, MAO inhibitors and benzodiazepines. Each treatment is limited, however, by adverse effects, risk of lethal overdose, and/or risk of drug dependence and withdrawal effects. There is a need for medications which are safer, and better tolerated. Fluoxetine is a recently marketed anti-depressant with a safer and milder side-effect profile compared to available anti-panic drugs. It has appeared highly promising in open trials for panic disorder, and requires testing in a placebo and standard- drug controlled study. There has also been growing recognition that pharmacotherapy for most patients with panic disorder is most effective when continued for 6 to 12 months. Long-term treatment, however, often leads to additional treatment emergent adverse effects, such as weight gain on tricyclics and MAOIs. Any putative anti-panic medication should be effective and well-tolerated in long-term treatment. The specific goals of this proposal are to compare the efficacy and adverse effect profile of fluoxetine to placebo and imipramine in a double-blind 8 week acute trial and 26 week maintenance phase for patients with panic disorder with or without agoraphobia. The long-term goal is to improve the clinical treatment of panic disorder by identifying new safer treatments, comparing them to, and integrating them with, standard treatments.